Tri-band terahertz vortex beam generator based on a completely independent geometric phase metasurface
Jiaqi Liu, Yongzhi Cheng, Fu Chen, Hui Luo, Xiangcheng Li
Abstract
A vortex beam carrying orbital angular momentum (OAM) is an effective way to increase wireless communication capacity. The existing terahertz (THz) metasurface (MS)-based vortex beam generator is limited to operating at a single or two frequency points, and also has wavelength-dependent behavior, thus limiting its application. Here, a transmissive MS is proposed to generate THz vortex beams carrying OAM with different topological charges <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>l</mml:mi> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>2</mml:mn> </mml:mrow> </mml:math> , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>3</mml:mn> </mml:mrow> </mml:math> ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>l</mml:mi> <mml:mo>=</mml:mo> <mml:mo>+</mml:mo> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>+</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>2</mml:mn> </mml:mrow> </mml:math> , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>+</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>3</mml:mn> </mml:mrow> </mml:math> ) under left/right-handed circular polarization (CP) incidence at three different frequencies. The mode purity of the generated vortex beam with topological charges of all orders is greater than 60% under CP wave incidence at 0.6 THz, 0.857 THz, and 1.892 THz, respectively. Both theoretical prediction and simulation results verify that the designed MS can independently manipulate CP waves at three different THz frequencies. The proposed unit-cell structure is preferable for increasing the number of operation frequencies while guaranteeing the manipulation of electromagnetic waves. The compact system of unit-cell structure provides ideas for the fabrication of multifunctional THz MS-based devices with higher integration and smaller size.